1. Field of the Invention
The present invention relates to a method for recovering metals from wastes. In particular, the present invention relates to a method for recovering re-usable metals from waste electronic components, and vitrifying the waste residue using microwave energy.
2. Discussion of Background
The proper disposal of all kinds of wastes is an important issue. In particular, the disposal of hazardous wastes: biological, chemical and radioactive, is of concern to generators, regulatory officials and the public. These waste materials can present a hazard if they re-enter the environment. Furthermore, there is the related issue of reduction of-waste volume and minimization of disposal space. Although progress has been made in reducing the volume of wastes generated and in recycling some components of the wastes, there remains a large volume of material that must be safely disposed of.
Many solid wastes, including waste electronic components such as used circuit boards, vacuum tubes, transistors, relays, wiring, television screens and computer monitors, remote controls, personal computers and calculators, contain metals, organic compounds and potentially leachable constituents. With the ever-increasing use of electronic devices in our society, there is a growing interest in the safe disposal of these types of wastes. Furthermore, even though the amount of metal in a single discarded electronic component is small, many millions--perhaps billions--of such components are discarded each year. This number is increasing along with the growing number of electronic devices in business, industry, military and household use. In the U.S. alone, such wastes may contain many thousands of pounds of potentially useful, valuable metals, including gold, silver and copper. There are no known, routinely-used, cost-effective methods for recovering the various metals found in a variety of waste electronic components.
A number of techniques have been used in stabilizing and encapsulating hazardous wastes and the literature abounds with descriptions of these. A particularly effective technique, called vitrification, is the encapsulation of wastes in glass. Glass is very stable against chemical attack. Vitrification has been studied for decades in connection with radioactive wastes. Typically in vitrification, the waste is slurried with glass frit into a glass melter where the glass is heated until it is molten. The waste is incorporated into the glass matrix in such a way that the final, cooled product will resist leaching of the waste for very long periods of time.
In other applications of vitrification, electrodes are placed directly in contaminated earth, which typically has a significant silicate component, and a voltage applied. The resistance of the ground results in sufficient joule heating to vitrify the waste in-situ.
Heat can be applied to wastes using a variety of electrical and thermal heating processes. The use of electric melting concepts is well known for incorporating waste into glass and the use of microwave energy is a favored technology for treating halogenated hydrocarbons. Varma (U.S. Pat. No. 4,935,114) brings toxic wastes into contact with a bed of non-metallic, absorbing particles, such as activated carbon, then heats the waste to 500.degree.-600.degree. C. to destroy the wastes chemically.
In all electrical and thermal waste treatment processes, there is a need for a method for recovering re-usable materials, including precious metals from waste electronic components. Preferably, the method should be simple, flexible and effective, reduce the overall volume of waste substantially, and produce a stable, durable product.